1. Field of the Invention
The invention relates to a method for calibrating a smart-antenna array of a time division duplex (TDD) system, which smart antenna comprises an array of at least two antenna elements, each with a transmit (TX) and a receive (RX) radio frequency branch. The invention equally relates to a radio transceiver unit with a smart antenna array for a wireless access system using time division duplex, and to a calibrating system for calibrating a smart antenna array of a wireless access system using time division duplex.
2. Description of the Related Art
Smart antennas are known to be employed as transmit and receive antennas in wireless access systems, in particular in base stations of such systems. Smart antennas, which generally use an array of antenna elements, enable the output of fully steerable beams patterns. They can be employed for example for a user specific digital beamforming, in which a beamformer of the smart antenna array is able to weight phase angle and/or amplitude of the signals transmitted by different antenna elements of the array in a way that the direction of the beam is adapted to move along with a mobile terminal through the whole sector of coverage of the antenna array.
Smart, or adaptive, antennas offer several benefits for wireless communications systems. The directivity of smart antennas can be used for example to reduce the delay spread of a radio channel. Moreover, the diversity of the antenna guards against fading. When employed in a base station, the output power of mobile terminals served by the base station can be decreased due to the spatial gain, which results in a longer battery lifetime. Array antennas also serve to increase the range of base stations, and the interference power to and from neighboring cells can be lowered considerably, thus improving the signal-to-interference ratio and the overall network capacity. Finally, mobile terminals served by the same base station can be identified according to their spatial signatures. This enables to further increase capacity of the base station by serving several terminals in the same time slot in space division multiple access (SDMA) operation.
Since the smart-antenna algorithms are frequently implemented at the baseband, most smart antenna systems provide for each antenna element separate radio frequency (RF) components for transmission and for reception of signals in dedicated transmit and receive branches. The RF components of the different receive and transmit branches of the different antenna elements usually have different properties. If the differences between the antenna elements and between the different branches of each antenna element are not compensated by an array calibration, the gain of the smart antenna is degraded. Furthermore, these properties evolve differently in time, which makes a real-time calibration necessary.
Smart antennas can be employed e.g. in a time division duplex system, in which adaptive antennas are particularly effective, since the optimum pattern formed in reception mode can be used for determining an optimum transmission pattern. The properties of such a TDD system can be made use of for calibration. In a TDD system, the employed frequency channels are divided into time slots, which are alternatingly reserved for transmission in uplink and downlink direction. Therefore, there is always only one direction of transmission possible at a time. The TDD is realized by connecting alternatingly a transmit and a receive branch in each antenna element.
Kentaro Nishimori, Keizo Cho, Yasushi Takatori, and Toshikazu Hori propose in xe2x80x9cA new calibration method of adaptive array for TDD systems,xe2x80x9d in Proc. Antennas and Propagation Society, IEEE International Symbosium, 1999, vol. 2, pp. 1444-1447, a method for calibrating a smart antenna array with a plurality of antenna element for TDD systems.
In this method, first a signal transmitted via the transmit branch of a first antenna element is divided by a directional coupler in the transmit branch. The signal is received via a switchable connection by the receive branches of each of the other antenna elements in turn. Then, signals transmitted via the transmit branches of each of the other antenna elements are divided by directional couplers in the respective transmit branch. Signals divided from one of these transmit branches after the other are received by the receive branch of the first antenna element by switching between corresponding connections. For the respective transmitted and received signals, the phase differences and gains are determined, which are then used for calculating for each antenna element a calibration value that can be used for correcting at the same time phase and gain in relation to a selected reference antenna element. Phase calibration and amplitude calibration are therefore combined. Due to the fact that the method is based on a TDD system, no signals from outside of the unit comprising the smart antenna array are received during the transmissions by the antenna elements. Therefor, the evaluated received signals result exclusively from the respectively evaluated transmitted signal of the antenna array.
It is an advantage of this method that in contrast to other known calibration methods, the calibration can be carried out during normal transmissions, using the signals transmitted during these normal transmissions as calibration signals. Therefore, no additional signal generator is needed and no transmission capacity is lost.
A disadvantage of the described calibration system, however, results from the switching used for selecting different antenna elements of the antenna array. The performance of active or semi active components like switches is varying in time, the consequence of which are unstable calibration results. Furthermore, an implementation using switches can be rather expensive.
It is an object of the invention to provide a method, a radio transceiver unit comprising a smart antenna array and a calibration system that enable a time stable calibration of a smart antenna array.
This object is reached on the one hand with a method for calibrating a smart-antenna array of a wireless access system using time division duplex, which smart antenna array comprises at least two antenna elements, each with a transmit and a receive radio frequency branch. The calibrating is based on the evaluation of signals transmitted via the transmit branch of one of the antenna elements and received via the receive branch of another one of the antenna elements respectively. The transmit and receive branches used for the respective transmission are physically interconnected. At least those transmit branches of antenna elements connected to a receive branch of an antenna element that is connected at the same time to a transmit branch of at least one other antenna element are employed in a predetermined order for transmitting broadcast messages in predetermined broadcast periods during which the respective other antenna elements are prevented from transmitting signals.
On the other hand, the object is reached with a radio transceiver unit for a wireless access system using time division duplex comprising a smart antenna array with at least two antenna elements, each including a transmit and a receive branch. In the transmit and the receive branch of each antenna element a directional coupler is provided for coupling signals out of the transmit branches and for coupling signals into the receive branches. Each coupler in the transmit branch of an antenna element is connected by a physical connection to the coupler in the receive branch of at least one other antenna element. Selection means are provided for selecting in a predetermined order at least each of those transmit branches of antenna elements connected to a receive branch of an antenna element that is connected at the same time to a transmit branch of at least one other antenna element to be employed for transmitting broadcast messages in predetermined broadcast periods during which the respective other antenna elements are prevented from transmitting signals. Finally, processing means are employed for evaluating signals transmitted via said connections for obtaining calibration information to be used for calibrating the smart antenna array.
Finally, the object is reached with a calibrating system for calibrating a smart antenna array with at least two antenna elements of a wireless access system using time division duplex, each antenna element including a transmit and a receive branch. The proposed calibration system comprises directional couplers, connections, selection means and processing means corresponding to those of the proposed radio transceiver unit.
The invention proceeds from the idea that in wireless access systems, usually broadcast periods are provided, in which periods the same information is transmitted to several recipients, and in which the regular point-to-point smart-antenna operation cannot be used. In situations where the element radiation patterns of the array cover the same angular space as the radiation pattern of the whole array, it is known to use only a single one of the antenna elements of the smart antenna array for transmitting broadcast messages during the predetermined broadcast periods. If in contrast and in accordance with the invention, the antenna element employed for transmitting the broadcast messages is varied, a receive branch of an antenna element can be connected in a fixed way to the transmit branches of up to all other antenna elements taking their turn in transmitting the broadcast message. During each broadcast period, the receive branches of the antenna elements of a smart antenna array in a TDD system then receive at the most a signal from the transmit branch of the antenna element currently used for transmitting the broadcast message. Thus, switches in the calibration system are not necessary. Avoiding switches results in an improved calibration since only passive components are required, the properties of which practically do not change in time. Moreover, an expensive implementation of switches is avoided.
At the same time, there is no need for a dedicated generation of calibration signals and for the provision of a time period that can be used exclusively for calibration, since the signals transmitted anyhow as broadcast messages can be used as basis for calibration measurements, and, depending on the configuration, possibly partly as well signals transmitted during smart antenna operation.
Preferred embodiments of the invention become apparent from the subclaims.
In a first preferred embodiment, transmitted and received signals are evaluated by determining only the phase difference between them. These phase differences are then used for determining a value for a phase calibration of each antenna element. Phase calibration alone can be sufficient, if only the power received by or from some other radio transceiver unit is to be maximized. From the implementation point of view, doing phase calibration only might in some situations be easier.
In an advantageous implementation of the invention, a value for a phase calibration for each antenna element is determined by fixing a calibration phase for one of the N antenna elements to a value {circumflex over (B)}1 and by adjusting the phases of the Nxe2x88x921 further antenna elements with calibration phases {circumflex over (B)}2 to {circumflex over (B)}N determined relative to the fixed value. For the transmitted and received signals, the respective phase difference xcex94ij is then determined. The first index identifies the respective transmitting and the second index the respective receiving antenna element. Both indices i,j lie between 1 and N. For a particularly simple determination of calibration phases, pairs of phase differences xcex94ij, xcex94ji are determined, each antenna element being represented in at least one of these pairs. For example, it would be sufficient to determine the phase differences xcex941j, with j=2 to N, of signals transmitted by the first element and received by each of the other elements, and the phase differences xcex94i1, with i=2 to N, of signals received by the first elements after having been transmitted by each of the other elements.
In a next step of this implementation, calibration phases {circumflex over (B)}2 to {circumflex over (B)}N are determined based on the one hand on suitable equations {circumflex over (B)}jxe2x88x92{circumflex over (B)}i={circumflex over (x)}ixe2x88x92{circumflex over (x)}jxe2x88x92(ŷixe2x88x92ŷj), the hat {circumflex over ( )} denoting the phase of the respective variable, wherein xi is the transfer function of the transmit branch of the ith antenna element and wherein yj is the transfer function of the receive branch of the jth antenna element, and based on the other hand on suitably determined phase differences xcex94ij, the phase differences xcex94ij corresponding to {circumflex over (x)}i+ŷj and the indices i,j lying between 1 and N. In case pairs of phase differences xcex94ij, xcex94ji and were determined as proposed above, for example, the solving of equations {circumflex over (B)}jxe2x88x92{circumflex over (B)}i=xcex94ijxe2x88x92xcex94ji immediately results in the desired values for the phase calibrations of the different antenna elements.
In another preferred embodiment, transmitted and received signals are evaluated by determining alternatively or in addition to the phase differences the gains of the signals on the respective transmission path. These gains can be used for determining a value for an amplitude correction for each of the antenna elements.
An amplitude calibration is needed in particular, if the smart antenna array is to be calibrated for nulling, i.e. for placing nulls in the radiation pattern of the array for some directions.
In a preferred embodiment of the invention in which an amplitude calibration is to be carried out, first, transmitted and received signals are evaluated by determining the gain xcex94ija of the respective received signal compared to the respective transmitted signal. The first index identifies the respective transmitting antenna element and the second index identifies the respective receiving antenna element, both indices i,j lying between 1 and N, wherein N is the number of the at least two antenna elements of the array. Then, factors |Bn|, with n=1 to N, are determined for an amplitude correction for each of the N antenna elements based on the determined gains xcex94ija and on the equations:             "LeftBracketingBar"              B        n            "RightBracketingBar"        =                            "LeftBracketingBar"                      x            1                    "RightBracketingBar"                ⁢                  xe2x80x83                ⁢                  "LeftBracketingBar"                      y            n                    "RightBracketingBar"                                      "LeftBracketingBar"                      x            n                    "RightBracketingBar"                ⁢                  xe2x80x83                ⁢                  "LeftBracketingBar"                      y            1                    "RightBracketingBar"                      ,      xe2x80x83    ⁢      n    =          1      ⁢              xe2x80x83            ⁢      to      ⁢              xe2x80x83            ⁢              N        .            
Corresponding to the proposed equations for determining values for the phase calibration, xn is the transfer function of the transmit branch of the nth antenna element, and yn the transfer function of the receive branch of the nth antenna element. Since each determined gain xcex94ija corresponds to |xi∥yj|, the equations can be solved easily, if suitable and sufficient gains xcex94ija are determined. More specifically, the gains xcex94ija are equal to |xi∥yj| except for some multiplicative values resulting from the amplitude of the transfer functions of components on the transmit path between a respective transmit branch and a respective receive branch. These transfer functions can be supposed to be known.
The employment of the invention can be seen in particular, though not exclusively, in wireless local area networks (WLAN), like IEEE 802.11a and HIPERLAN/2 (high-performance local-area network) networks, and in wireless rooftop routers (WRR).
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.